fix: Don't allow serialization of firestore settings

[abhishekwebcode]

When logging any firestore object like WriteBatch,Transaction,etc the settings object also gets logged / exposed
This can be seen by running JSON.stringify on any firestore object even a document reference
Many developers log firestore objects to help them debug testing/prod issues, this leaking of entire firestore key via this._settings is a bad practice as per me
We can also use Object.defineProperty to make it non-enumerable or any other technique that you like

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Fixes #<issue_number_goes_here> 🦕

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[ehsannas]

Thanks for your contribution @abhishekwebcode . If I understand correctly, you are concerned about users seeing the name of your Firestore host from the Settings object, is that correct?

[abhishekwebcode]

@ehsannas no i am worried about developers with log access seeing the firestore private key ( the one that initialises firebase-admin library nodejs )

[ehsannas]

@abhishekwebcode Can you please sign the CLA?

https://github.com/googleapis/nodejs-firestore/blob/main/CONTRIBUTING.md#contributor-license-agreements

[abhishekwebcode]

hey @ehsannas i have signed the agreement now

[MarkDuckworth]

There could be scenarios where the serialized settings object is useful. Could we a) make this behavior optional, b) redact only the key, or c) make the key non-enumerable?

[ehsannas]

Thanks for the suggestion. I have updated the code to only redact the credentials part of the settings object. PTAL @MarkDuckworth